This invention relates to a collision speed detecting sensor, and more particularly to a sensor suitable for detecting the collision speed of an automobile.
As a protective apparatus for relieving a shock to which an occupant is subject when an automobile collides, there is an air bag system. The system is equipped with means to detect the magnitude of an impulsive force at the collision.
The magnitude or severity of the shock which the occupant undergoes is concerned, not only with the magnitude of the impulsive force G, but also with the period of time in which the impulsive force G acts. That is, the magnitude of an impact energy which is generally represented by the product between the impulsive force G and the duration thereof is concerned with the magnitude of the shock which the occupant undergoes. From this fact, the magnitude u of the shock can be expressed by: ##EQU1## Here, t denotes the period of time, M the mass of the occupant, G the instantaneous value of the impulsive force, and .tau. the duration of G. It is considered from Eq. (1) that the magnitude u of the shock is proportional to the whole change of the car speed from the collision to the stop of the car.
For the above reason, a system which uses a sensor for detecting the whole change of the car speed at the collision of the car has been proposed as improvements in the air bag system. As the sensor of the improved system, there has been known one which employs a spring-mass system. More specifically, the spring-mass system is constructed of a conventional linear spring whose one end is fixed to a base structure and whose other end is a free end, an inertial mass body (hereinafter called "mass") which is attached to the free end of the spring, and an electric contact member. Normally, the mass lies at an inoperative position owing to the preload of the spring. In order that the magnitude of the shock to which the occupant is subjected may be foreknown as early as possible, the sensor is mounted at that part of the car at which the whole change of the speed at the collision of the car appears first, for example, in the vicinity of the front bumper of the car. When the whole change of the car speed is received, the mass moves against the spring force of the spring. When the magnitude of the whole change of the car speed is greater than a predetermined value, the mass is connected with the electric contact member, so that the system is endowed with an electric energy and is actuated. The waveform of the impulsive force G which is an input flowing into the sensor is substantially a half sinusoidal wave, and can be expressed by: EQU G = G.sub.p sin .omega..sub.o t (2)
Here, G.sub.p denotes the peak value of G, t the period of time, and .omega..sub.o the angular frequency of the input. Letting .tau. be the duration of G, .omega..sub.o = .pi./.tau.. Now, letting m be the mass of the inertial mass body and k be the elastic modulus of the spring, the amount of movement x of the mass becomes: EQU d.sup.2 x/dt.sup.2 = G.sub.p sin .omega..sub.o t - .omega..sup.2 x (3)
where .omega..sup.2 =k/m
.omega. denotes the natural angular frequency of the system consisting of the spring and the mass.
When .omega.=.omega..sub.o in Eq. (3), x diverges. Accordingly, the natural angular frequency .omega. need be made smaller than the angular frequency .omega..sub.o of the input G. In order to make .omega. small, the elastic modulus k of the spring is made small or the mass of the inertial mass body is made large. Thus, the amount of movement x of the mass is apparently determined. From experimental results of car collisions, the duration .tau. of the input G is below 30 (msec.). Then the optimum value of .omega. becomes about 100 (rad/sec.). The maximum amount of movement of the mass at this time becomes about 55 (mm) at a collision speed of 13 mph. Therefore, the sensor is large in size as one of this type and becomes expensive.